I have the speakers set as small and the sub on. What would be the correct crossover setting and should it be the same on the receiver and the sub? I believe the sub crosover can be set anywhere between 60 and 120 and the can be set at anywhere between 60 and 150 on the receiver.

There is also a "phase" switch on the backof the sub that i'm not sure what it does.

I looked up your speakers and it looks like they are rated down to 80Hz so I would try setting the the crossover between 100Hz and 120Hz. You always want to leave a little overlap so just play with it between those settings and see which sounds best. As you get closer to 120 you will may be able to localize the sub a bit more.

The phase switch will shift the phase of the subwoofers output 180 degrees. If you put your sub in certain locations, the distance between your sub and main speakers might be such that the sound waves will cancel out. Using this switch can shift the waves coming out of the sub to help alleviate this problem.

Sit in your listening position and have someone toggle the switch back and forth, and try to determine which sounds better.

Your mains won't cover low enough frequencies for phase to be an issue, so I wouldn't worry about it the phase relationship between them. A simple check would be to set it to 0 and 180 and see which sounds best, and if you can't tell the difference, leave it on 0.

For the x-over, leave it on 120 on the sub, since your receiver is handling the x-over already. I agree with Seth, I'd start out with about a 100Hz x-over and experiment within about 20Hz or so, to see what blends the best.

Try 100Hz first, because if you set the receiver to 120, you may cascaed the x-over. I have a feeling right around 100 will work for you, but I can't say for sure, as your room will be a factor, particularly for the sub.

No. You want to set the crossover on the sub as high as it will go, so that the receiver's crossover will take effect (but not the sub's). In other words, you want your receiver's crossover to be lower than your sub's (you don't want them to "match").

Basically, the lowest crossover of the two will be the one that affects the sound. In reality though, it's more complicated, especially if the two are close in proximity, because one may have a higher order filter than the other.

You don't happen to have an input on your sub that bypasses the crossover, do you? If so, I would use that.

I used to have MC300's for my L/C/R. What I found is that 110 was about perfect when using the low pass on the sub. I wanted to use the line level input however, and my receiver choices were 80-100-120. To my ears there wasn't much difference between 100 and 120 so I chose 100 and was very happy with it.
In one of your other posts you had mentioned entertaining the idea of upgrading to CSW bookshelves. I did that per John Garcia's recommendation a few months back, and have been absolutely thrilled. (Not that the MC300's are bad. For satellites, they were the best that I had heard out of many companies that I auditioned.)
By the way John, after extensive listening, I went with the M50's over the M60's. The manager pointed me in this direction and to my ears they sounded much more natural on both voices and instruments. They didn't jump out at me like the M60's did (Which I am sure some people would love), but after extended listening I liked the sound of the M50's. I guess it just goes to show you that different people like different qualities in speakers. Anyway, thanks for your help.

I think I will have to go audition the M50, M60 and M80s with the MC 400 as a center and the MC 300s as rears to see what my ears like - I think this is half the fun of it all - listening to all this stuff